Why was the Nazi-Soviet pact a key factor causing the Second World War?

The Nazi-Soviet Pact (also known as the Molotov-Ribbentrop Pact, after the representatives who signed it) was certainly a surprise when it was announced, but it had been in the works for longer than most people realise.
The terms of the treaty were fairly simple–at least the terms the world knew about. Germany and the USSR pledged mutual nonaggression and peace for a period of 10 years. There was, however, a secret protocol, whereby the dictators Hitler and Stalin agreed to split up most of Eastern Europe and the Baltic states (Estonia, Latvia, Lithuania) between them. The most important division was a line drawn through the center of Poland, with each country taking about half. Neither Germany nor the USSR much liked the idea of an independent Poland, which was thrust upon them after World War I by the Treaty of Versailles. The Nazi-Soviet Pact intended to ensure that the Poles would never have control of their own destiny.
Surprising as it was, the deal had a kind of logic to it. If Hitler was going to take over Poland–which was clear in the summer of 1939 that he desperately wanted–he had to make sure that his arch-enemy Stalin wouldn’t come in against him. On the other side of the table, Stalin needed the deal to buy time to arm himself against Hitler, who he was shrewdly certain would eventually attack him. Neither dictator had any expectation of keeping the treaty for very long. Hitler’s aim, as described in Mein Kampf in the 1920s, made a German attack on Russia inevitable. Some historians, such as Edvard Radzinsky, maintain that Stalin was planning to attack Hitler before Germany invaded the USSR on June 22, 1941.
One thing was certain: the announcement of the Nazi-Soviet Pact lit the final fuse that led to the outbreak of World War II. Hitler’s troops were already on the move at the time he sent Ribbentrop to Moscow. Barely more than a week later, on September 1, 1939, they crossed the border and invaded Poland. Great Britain, pledged to defend Poland, had no choice but to declare war. Within ten days after the pact was signed, the Second World War was on.